Fluid mounts of the "hydraulic damper" type have long been used in vehicular and other applications to dampen shocks and/or vibrations. A typical hydraulic damper has interconnected variable volume chambers between which hydraulic fluid passes during excitation of the mount. Resistance of the fluid to flow between the chambers opposes and damps vibratory and similar forces imposed upon the mount. The viscous damping forces generated by the mount are proportional to, among other things, the viscosity of the hydraulic fluid and the extent to which its flow between the chambers is "throttled" or otherwise impeded by the orifice or conduit through which the fluid passes. The use of hydraulic fluids of relatively high viscosity is therefore acceptable and desirable in many viscous fluid dampers.
A newer type of fluid mount, which has received increasing acceptance within recent years, utilizes fluid inertia forces to achieve and/or to enhance the desired attenuation of vibratory forces. A plot of the dynamic stiffness against the excitation frequency of mounts of the fluid inertia type typically includes a notch-like region, at which the dynamic stiffness of the mount is greatly reduced and may be considerably less than its static stiffness, followed by a "peak" of large dynamic stiffness. A mount may be so designed as to cause the foregoing abrupt variations in its dynamic stiffness to occur at a particular excitation frequency where a specific vibration problem exists. For example, objectional "drone" noise occurring within some automobiles as a result of transmission to their frames of engine firing vibrations generated at a particular engine speed, may be substantially eliminated by the use of an inertia type engine mount that is specifically designed so as to possess its minimum-stiffness "notch" at the frequency of the aforesaid vibrations.
While static mount tuning is satisfactory for the attenuation of troublesome vibrations occurring at only one particular frequency, problem vibrations such as those producing vehicle "drone" noise may occur at a number of significantly differing engine speeds and mount excitation frequencies. In such a situation it is highly desirable for a mount to be dynamically tunable so as to permit selective variation during mount operation of the frequencies at which the mount has very low dynamic stiffness. Since the frequency at which stiffness reduction occurs is a function of, among other things, the size of the fluid flow path between the variable volume chambers of the mount, one theoretically possible way of dynamically tuning the mount is by varying the flow path cross-sectional area. In a mount containing a plurality of flow passageways between the chambers, this result should be realizable by selective opening and closing of valve means associated with one or more of the passageways. However, the expense, complexity and/or relative slowness of operation of conventional mechanically or electromechanically actuated valves makes their use less than satisfactory for the foregoing purpose.
A possible alternative to the use of conventional valves and conventional hydraulic fluids, such as glycol and/or water, is the use of "valves" that generate electrical force fields and of fluids that undergo substantial rheological changes in the presence of appropriate electrical fields. Two types of field responsive fluids, whose use has heretofor been proposed in fluid mounts of the viscous damper type, are magnetic fluids and electrorheological fluids. While differing in various significant respects, magnetic and electrorheological fluids both increase in apparent viscosity (i.e. in the extent to which they resist flow under applied stress) when subjected to a magnetic field and to a high voltage electrical field, respectively. While such fluids may be satisfactorily utilized in fluid mounts that merely produce viscous damping, their use normally is not satisfactory in mounts that generate and utilize fluid inertia effects. Magnetic fluids customarily have high apparent viscosity even when not exposed to a magnetic field, and the particles therein retain their polarities for an appreciable time period after each exposure of the fluid to such a field. Consequently, the flow resistance or "drag" of the "unactivated" fluid is so great as to prevent the generation of any significant fluid inertia forces. While having a lower apparent viscosity in their unactivated state, as well as being superior in other respects to magnetic fluids, the apparent viscosity of deenergized electrorheological fluids still will usually be some twenty or more times greater than that of the water and/or glycol fluids customarily utilized in inertia type mounts. The problem presented by the greater apparent viscosity of electrorheological fluids is compounded by the fact that the field-producing electrodes of the valve between which the fluid passes must usually be spaced closely to one another if the fluid is to withstand, without shear and when activated by the electrical field, a large magnitude pressure differential across the valve. The resistance of the fluid to flow through the spaces or gaps between the valve electrodes is inversely proportional to the cube of the width dimension of the gap. Attempted utilization of closely spaced electrodes which extend along all or most of the length of the or each fluid passageway between the mount chambers, which passageway is customarily relatively long in inertia type mounts, results in such high flow resistance and reduced flow as to impede generation of inertia forces of the desired magnitude. Consequently, the desired abrupt decrease in mount dynamic stiffness is not realized during mount operation.